Perfect Amulet

The Perfect Amulet (パーフェクトアミュレット) is an ornate amulet which serves as a key to the Temen-ni-gru's gate to the Demon World, and is also necessary to awaken the Force Edge. A large red gem is set into the amulet, and it can be split into two halves. It was given as a gift to Eva by Sparda, and she gave the halves to their twin sons , Dante and Vergil, on their birthday. Their names are engraved on the back of each half.

History
Originally, the Perfect Amulet was the key that allowed one to enter into the Demon World; a gift that the Demon King gave to the corrupted humans who wanted power. After Sparda's rebellion, Sparda used the amulet along with his own blood and that of a human priestess to seal off the two worlds. Centuries later, Sparda encounters Eva, who he then gives the amulet to. After his death, Eva gives the two halves of the amulet to Vergil and Dante, her twin sons with Sparda, as their eighth birthday present.

Devil May Cry 3
Vergil comes after Dante for his half of the amulet, in order to attain their father's power and unseal the Temen-ni-gru's gate to the Demon World. Eventually, Vergil takes hold of Dante's half of the amulet after a battle atop the Temen-ni-gru. After a betrayal by Arkham, who manages to open the gate by stealing both halves, Dante and Vergil defeat him, sending him back to the Human World. In a climactic battle, Dante and Vergil each retrieve their halves, and Dante returns to the Human World, while Vergil stays behind.

Devil May Cry
Years later, on Mallet Island, Dante still wears his half around his neck and Vergil, now Nelo Angelo, retains his half, although it is hidden from view. After the first battle with Dante, Nelo sees Dante's half, and begins to react in pain at the sight of it. This is most likely Vergil's memories surfacing, affecting Mundus's control over him. In the final battle between the two brothers, Dante defeats Vergil, taking his half to enter the Demon World and finish off Mundus. At the end of the game, it can be seen around Trish's neck. It seems Dante has entrusted it to her, though the amulet has not been seen or mentioned since.

Trivia

 * In Devil May Cry, both halves of the Perfect Amulet are wrought in silver, while in Devil May Cry 3, Dante's half is wrought in silver while Vergil's is wrought in gold.
 * The Proud Soul which serves as a currency in Devil May Cry 4 resembles the Perfect Amulet, but shattered.